The only reason I'd pay to see this fight is to see Diaz shut up once and for all. I personally don't even think the guy could take Claude P, Koscheck, Diego, Fitch, Marquardt, Penn, Condit, or Rory. But hey, let's see him fight GSP who has walked through everyone else in the WW division.
For a casual fan this fight makes no sense, another semi no name like Shields coming in because the UFC has drained the WW division of contenders. GSP has a very few interesting fights left so I suppose this guy is as good as any other. I think he'll get wrecked and I'd love to see it.
Condit, Marquardt (maybe), Rory (maybe), Diego (heh why not) and Claude P are the only guys in the WW division I'd like to see GSP fight. Diaz vs GSP is in no way a superfight.